To complete the square for the polynomial \( p^2 - 18p \), we will follow these steps:
- Take the coefficient of the linear term, which is \(-18\).
- Divide it by 2: \[ -18 \div 2 = -9 \]
- Square this result: \[ (-9)^2 = 81 \]
Therefore, the number that makes the polynomial a perfect-square quadratic is 81.
Thus, the completed square form of the polynomial will be: \[ p^2 - 18p + 81 = (p - 9)^2 \]